This invention pertains to a method and apparatus for determining a person's flicker fusion threshold. When a person observes light pulses which are increasing in frequency, at some point it appears to the person that he is observing a continuous light. That point is known as the flicker fusion threshold. The instant method and apparatus provides such varying light pulses in the presence of either continuous or flickering background light. By so doing, with the latter method, the incidence of detection of abnormal flicker fusion thresholds in persons having neurological deterioration of their optic pathways is increased.
In the past, apparatus for detecting the flicker fusion threshold or critical flicker fusion (CFF) frequency as it is known, included only a flickering target light, the frequency of which is adjustable. Typically, a person being tested increases the frequency of the flickering target light to the point at which it appears to be continuous, such frequency being that person's CFF frequency.
The instant invention includes means for providing a flashing background light in the presence of the flickering target light. The person being tested increases the frequency of the target light to the point where it appears as if a non-flickering light. The frequency of flickering at that point is referred to herein as the incremental critical flicker fusion (ICFF) frequency.
It is well known that several diseases, i.e., multiple sclerosis, diabetes and glaucoma involve neurological deterioration. Such deterioration can be detected and the status of neurological function can be monitored by use of both the CFF and ICFF test. As a disease having neurological involvement with the optic pathways becomes more severe, both the CFF and ICFF for that person decreases.
Another past apparatus for measuring CFF frequency includes a central flickering light surrounded by a steady background light, both of such lights being covered by an opal glass which diffuses the boundary between the two lights. The central flickering light is varied (as in past apparatus with a single flickering light) to the point at which the person being tested observes what appears to be a continuous light.
A general object of the present invention is to provide both an improved and a unique method and apparaus for obtaining fusion threshold measures.
A related object is to provide improved detection of visual disfunction.
According to a preferred embodiment of the invention, a translucent viewing panel is provided, such having a white incandescent background light mounted behind it. A red light-emitting diode is mounted on the front of the screen in the center of an opaque cylinder which encloses the diode. The cylinder is mounted on the viewing panel. The diode's leads pass through a small hole in the panel and are connected to a voltage-controlled oscillator. The background light is connected to either a different oscillator, such switching the background light on and off at a frequency of 10 cycles per second or to a continuous power source. A pair of lenses positioned in front of the viewing panel focus the exiting light into parallel rays. A hood is provided to exclude all other light from the viewing panel. Additionally, the hood is provided with an aperture into which a test subject views the panel. Also provided are colored filters positionable between the aperture and the viewing screen for selectively varying the light entering the eye, and, accordingly, varying the population of fibers within the retina which react to the light.
A potentiometer varies the frequency of the voltage-controlled oscillator and accordingly varies the frequency of flashing of the light-emitting diode. A digital display indicates the frequency at which the light-emitting diode flashes. The test subject increases the frequency to the point at which the light-emitting diode appears to be on continuously; the frequency displayed at that point is the fusion threshold frequency for which the instant apparatus is constructed to detect. For the instant apparatus, the above-described measurement is called the incremental critical flicker fusion (ICFF) frequency (or threshold).
Other features and advantages offered by the invention will become more fully apparent as the description which follows is read in conjunction with the accompanying drawings.